1. Field of the Invention
The present invention relates to a high-pressure fuel pump for a cylinder injection type engine, which high-pressure fuel pump has a pulsation absorber and, more particularly, to a high-pressure fuel pump equipped with a pulsation absorber which is provided as an integral part of the high-pressure fuel pump and which absorbs the pulsation at the low pressure end thereof.
2. Description of Related Art
A diesel engine has been widely known as an engine designed to inject fuel in the cylinders of the engine which is referred to as a cylinder injection engine or a direct injection engine. In recent years, the cylinder injection type has been proposed also for a spark ignition engine or a gasoline engine. In such a cylinder injection engine, a fuel pressure of approximately 5 MPa, for example, is necessary because the fuel is injected into a cylinder during the compression stroke of the cylinder, whereas the fuel pressure is approximately 0.3 MPa in the case of a conventional engine wherein a fuel-air mixture is produced outside a cylinder.
To obtain such a high fuel pressure, a high-pressure fuel pump is generally provided on the side of a fuel injector in addition to a low-pressure fuel-pump provided in a fuel tank. In general, the low-pressure fuel pump is driven by, for example, a motor or the like and it is driven at all times as long as the power is ON, while the high-pressure fuel pump is driven by an engine and it runs as the engine runs. The high-pressure fuel pump is provided with a pulsation absorber to absorb the pulsation that takes place in the pipe at the low pressure end so as to stabilize the discharge of the high-pressure fuel pump.
FIG. 11 is a side view illustrating a conventional high-pressure fuel pump, a part thereof being shown in a sectional view; and FIG. 12 is a system diagram of the pulsation absorber on the low pressure end. In the drawings, a high-pressure fuel pump assembly 100 has a casing 1, a cylinder 30 being provided at the bottom of the casing 1; and a plunger 31 is provided in the cylinder 30 such that it is able to reciprocate therein. The cylinder 30 and the plunger 31 constitute a fuel pressurizing chamber 32.
Formed on one side surface of the casing 1 is an inlet port 14 to which a low pressure pipe (not shown) extending from the low-pressure fuel pump is connected. An inlet passage 2 is formed between the inlet port 14 and the fuel pressurizing chamber 32; a filter 8 is provided at the boundary of the inlet port 14 and the inlet passage 2. The fuel supplied from the low-pressure fuel pump is fed into the fuel pressurizing chamber 32 through the inlet passage 2. Formed also on one side surface of the casing 1 is a discharge port 34 to which a high pressure pipe (not shown) extending to a fuel injector is connected. A discharge passage 35 is formed between the discharge port 34 and the fuel pressurizing chamber 32; the fuel which has been pressurized in the fuel pressurizing chamber 32 passes through the discharge passages 35 to be discharged outside. A resonator 36 is provided in the middle of the discharge passage 35.
The plunger 31 reciprocates in the cylinder 30; it takes fuel into the fuel pressurizing chamber 32 where it pressurizes the fuel, then discharges it outside through the discharge passage 35. The high-pressure fuel pump assembly 100 is a single-cylinder type which has the single cylinder 30. Hence, oil impact occurs at every intake or discharge operation in the inlet passage 2 and the discharge passage 35, causing the fuel to pulsate. In particular, the pulsation taking place in the inlet passage 2 causes the outflow of the high-pressure fuel pump assembly 100 to drop and also causes the low pressure pipe connected to the inlet port 14 to vibrate, producing noises.
Formed on the other side surface of the casing 1 is an approximately cylindrical recessed section 13; the outer edge of the bottom surface of the recessed section 13 is in communication with an inlet passage 2a coming from the inlet port 14. An inlet passage 2b extending to the fuel pressurizing chamber 32 is in communication with the central portion of the bottom of the recessed section 13. The inlet passage 2a and the inlet passage 2b make up the inlet passage 2. In the recessed section 13, an approximately cylindrical sleeve 15 is disposed, the outer peripheral surface thereof being sealed with an O ring 16. A support 17 forming an oil passage and a sealing member 18 for sealing the gap between a piston 20 and the sleeve 15 when the engine stops are placed between one end of the sleeve 15 and the casing 1. The support 17 has a cylindrical section 17a and a jaw 17b which extends outward in the radial direction from one end of the cylindrical section 17a; the cylindrical section 17a is in communication with the inlet passage 2b, and the jaw 17b has a through hole 17c through which fuel passes. An annular passage 19 is formed between the jaw 17b and the bottom of the recessed section 13. The sealing member 18 has a thin annular shape; it provides sealing between one end of the sleeve 15 and the support 17.
Slidably provided in the sleeve 15 is the bottomed cylindrical piston 20, the bottom thereof facing the casing 1. An extremely small gap is formed between the sleeve 15 and the sliding surface of the piston 20; the gap is filled with fuel to protect the sliding surface. The sleeve 15, the piston 20, and the support 17 make up a capacity chamber 25, which is a surrounded space. The portion of the sleeve 15, which portion juts out of the casing 1, is covered by a cup-shaped cover 21, the open end of the cover 21 being secured to the casing 1 to fix the sleeve 15 to the casing 1. Provided at a predetermined position of the cover 21 is a drain nipple 22 for returning the fuel, which has leaked out, back to a fuel tank (not shown). A spring 23 is provided in a compressed state between the bottom of the piston 20 and the cover 21. When no fuel pressure is applied, the restoring force of the spring 23 pushes the piston 20 to the right in FIG. 11 to hold it there.
In the high-pressure fuel pump having the configuration explained above, the pulsation absorber is provided somewhere in the middle of the inlet passage 2 of fuel of the high-pressure fuel pump to move the piston 20 to absorb the pulsation of the fuel according to the fluctuation in the fuel pressure. More specifically, the fuel supplied through the inlet passage 2a passes through the annular passage 19 and the through hole 17c into the capacity chamber 25, then goes to the high-pressure fuel pump through the inlet passage 2b. At this time, the fuel pulsates in the intake passage 2b due to the intake and discharge operation of the high-pressure fuel pump; when the fuel pressure is high, the piston 20 moves to the left in FIG. 11, while it moves to the right in FIG. 11 when the fuel pressure is low. Thus, the pulsation of the fuel in the inlet passage 2 is absorbed.
The cylinder injection, high-pressure fuel pump is characterized by the considerably high fuel pressure, approximately 5 MPa, and a wide range of pulsation including high-frequency pulsation. In the conventional piston-type pulsation absorber described above has poor responsiveness due to the frictional resistance between the piston 20 and the sleeve 15 and also to the dead weight of the piston 20. This has been posing a problem in that the pulsation in a high-frequency range including a surge pressure cannot be fully removed.
The conventional piston-type pulsation absorber on the low pressure end has been disadvantageous in that it is large and has many components with resultant high cost.